Gas turbine engines operate at high temperatures which may be greater than the melting point of the component parts of the gas turbine engine. These components require cooling during the operation of the gas turbine engine. There are some instances where a gas turbine engine will be working to a higher than normal capacity (for example, during take-off of an aeroplane if the engine is an aero engine) and components which do not usually require cooling in normal operation, will require cooling. Consequently, it is desirable to provide a means for modulating the flow of fluid to these components.